HOT-MELT ADHESIVES COMPRISING A BIMODAL POLYMER COMPOSITION FORMED BY POLYOLEFINS HAVING LOW STEREOSPECIFICITY

摘要

The present invention discloses novel hot-melt adhesive formulations, that show a novel and excellent processability, even in processes at high line-speed and at a relatively low temperature, said formulations being especially suitable for strongly bonding substrates that have openings, holes, voids or pores, both macroscopic and microscopic ones, like perforated plastic films, both with a bidimensional or a tridimensional structure; fibrous substrates, both woven and non-woven; porous plastic films, and the like, all said mentioned substrates being widely used in particular for the manufacturing of absorbent hygienic articles. Said new hot-melt formulations comprise, as their main or even unique constituent, at least one polymer composition formed by two polyolefins, substantially non-stereospecific, as showed by the low value of their crystallization enthalpy at time zero, said polyolefins having average Number Molecular Weights Mn that do not differ for more than about 6,000 g/mole, being the single average Number Molecular Weights of said polyolefins ranging from about 3,000 g/mole and about 30,000 g/mole. The hot-melt adhesive formulations according to the present invention show a crystallinity at time zero that is particularly low, as highlighted by their low initial Crystallization Enthalpy from the melt that is not greater than about 12 J/g when measured by DSC at a cooling rate of 1°C/minute. Moreover these adhesives have a low Solidification Rate around their rheological setting point, that is not greater than about 5,000 Pa/°C; and a low Decreasing Rate of their rheological parameter Tan Delta, still around the same setting point, that typically is not greater than about 0,5 (°C)-1 The described hot-melt adhesive formulations, upon aging in time, show also a significant variation of their crystalline content and of their main rheological parameters that further improve the already excellent initial adhesive strength and cohesion. In particular, in aged conditions, i.e. after seven days of aging at 23°C and 50% relative humidity, their low initial crystalline Enthalpy increases of at least about 50%, their Elastic Modulus at 23°C and 1 Hz also shows an increase also of at least 50% over the analogous G' at time zero; while their Tan Delta, again at 23°C and 1 Hz, has an aged value that is at least 30% lower than the initial value of the same Tan Delta. In a preferred embodiment, the hot-melt adhesive formulations according to the present invention are substantially free from low molecular weight additives, like tackifiers, traditional liquid plasticizers or waxes. In another preferred embodiment, said hot-melt adhesive formulations are, more in general, substantially free from polymeric and oligomeric compounds that derive from monomers that are different from mono-olefins from C2 to C6, and in particular free from poly-unsaturated monomers, cyclic or poly-cyclic monomers and in general monomer heavier than C6, whose unreacted residues may be toxic, sensitizing or allergenic, like for example isoprene, piperylene, limonene and other terpenes, phenol, indene, coumarone, styrene and its derivatives, abietic acid and its derivatives, various cyclic and polycyclic compounds etc.